The present invention relates to a method of transporting workpieces in an assembly line in which at least two kinds of workpieces are transported by mixture.
Conventionally, it is normal practice to transport workpieces in an automobile assembly line or the like at a fixed speed and a fixed pitch. For example, there is known in Japanese Published Unexamined Patent Application No. 64522/1992 a method in which an endless conveyor is loaded or charged with pallets on each of which is placed a workpiece in the form of an automobile body, and in which the timing for loading or charging the endless conveyor with pallets is controlled in accordance with the lengths of the automobile bodies on the pallets so that, when the automobile bodies are transported by the conveyor at a fixed speed, the distance between the rear end of a preceding automobile body and the front end of a succeeding automobile body, i.e., the transporting pitch of the workpieces, always becomes constant even when two kinds or more of automobile bodies of different lengths are transported by mixture. When plural kinds of workpieces are transported by the assembly line, workers are allocated to suit such workpieces as will require the maximum number of assembling man-hours.
In the above-described example, the number of workers becomes superfluous when the workpieces to be treated in the assembly line are changed over to those requiring relatively smaller number of assembling man-hours and, consequently, it gives rise to a problem in reducing the product costs.
As a solution to this kind of problem, the following may be considered. Namely, each workpiece is transported by a self-propelled transporting member and, when a workpiece requiring relatively smaller number of man-hours is treated, the speed of transportation is increased so that the problem of superfluous workers can be solved through consequent increase in the number of production. However, at an initial time when the kind of workpieces has been changed over, the transporting members for the succeeding workpieces which require smaller number of assembling man-hours will catch up with the transporting members for the preceding workpieces which require larger number of assembling man-hours, with the result that the workpieces will be accumulated or get stacked. Therefore, the problem of superfluous workers cannot be solved.
In view of the above problems, the present invention has an object of providing a method of transporting workpieces in an assembly line in which at least two kinds of workpieces are transported by mixture so that the transportation can be performed effectively without giving rise to a problem of superfluous workers.
According to the present invention, for achieving the above object, there is provided a method of transporting workpieces in an assembly line in which at least two kinds of workpieces are transported by mixture, each of the workpieces being transported by a self-propelling transporting member, the method comprising the steps of: setting in advance a transporting pitch for each kind of workpiece based on a difference in the number of assembling man-hours required for each thereof; and controlling a timing for starting the transporting member off a starting end of the assembly line according to the transporting pitch such that a pitch between a preceding transporting member and a succeeding transporting member becomes equal to that which is set for a workpiece to be transported by the preceding transporting member.
By controlling the timing for starting the transporting member at the starting end of the assembly line, the pitch of one transporting member (i.e., a preceding transporting member) and a succeeding transporting member which respectively travel on the assembly line, i.e., the transporting pitch of the workpieces is varied. Here, if an arrangement is made such that the smaller is the number of assembling man-hours of the kind of workpieces, the narrower becomes the transporting pitch, the number of production becomes larger with the workpieces requiring smaller number of assembling man-hours. Therefore, the assembling work can be effectively performed without giving rise to the problem of superfluous workers. Further, there is no disadvantage in that, at the time of changing over the kind of workpieces, the succeeding workpieces catch up with the preceding workpieces, with the result that the workpieces becomes accumulated or get stacked.
In addition, in case an automated working station having therein an automated device is provided in an intermediate portion of the assembly line, the transporting member is stopped at the automated working station to perform the assembling works on the workpiece by the automated device, and the transporting member is thereafter re-started. If this timing for re-starting is controlled according to the transporting pitch which is set in advance, the manual assembling works in the rear stage of the automated working station can be as effectively performed as in the above-described example.
As the method of determining the pitch between a preceding transporting member and a succeeding transporting member, there are the following two ways. Namely, one way is to adjust the pitch to the transporting pitch which is set relative to the workpiece to be transported by the succeeding transporting member. The other way is to adjust the pitch to the transporting pitch which is set relative to the workpiece to be transported by the preceding transporting member. According to the former way, if the workpiece to be transported by the preceding transporting member is, for example, the kind requiring larger number of assembling man-hours and if the the workpiece to be transported by the succeeding transporting member is the kind requiring smaller number of assembling man-hours, the pitch between the preceding workpiece and the succeeding workpiece becomes narrow. As a result, when workers perform assembling works to the workpiece while they move together with the transporting member from the starting end to the terminal end of each working region of the assembly line, the succeeding transporting member will enter their own working region over the starting end thereof. It follows that the position at which the assembling works are to be started varies with the kind of workpieces. On the other hand, according to the latter way, the pitch between the preceding workpiece and the succeeding workpiece is enlarged according to the workpiece that is transported by the preceding transporting member and that requires larger number of assembling man-hours. Therefore, when the workers have returned to the starting end of their own working region after finishing assembling works while moving together with the preceding transporting member, the succeeding transporting member will just have reached the starting end thereof. It follows that, even if the kind of workpieces is changed over, the position at which the assembling works are to be started does not vary. Therefore, the position for feeding parts at each working region needs not be varied depending on the kind of workpieces, resulting in an advantage in view of material handling or transporting of the parts. In this case, the timing for starting the transporting member can be automatically controlled to suit the preceding workpiece by: providing each of the transporting members or the workpieces with transportable identifying means which identifies the kind of the workpieces; attaching to the transportable identifying means data representing the kind of a present workpiece to be presently transported by any of the transporting members as well as data representing the kind of a preceding workpiece to be transported by the preceding transporting member; reading out with a sensor those data relating to the kind of the preceding workpiece which are attached to transportable identifying means mounted on a transporting member that is waiting for starting or to a workpiece mounted thereon; and controlling a timing for starting the waiting transporting member according to a transporting pitch which is set for the kind of workpiece whose data are read out.
Furthermore, when the transporting pitch is enlarged, some of the transporting members become superfluous. In such a case, the change in the required number of transporting members as a consequence of the change in the transporting pitch can be freely dealt with by providing stock means which can take in or take out transporting members to and from a return path which extends from a terminal end to a starting end of the assembly line, and by stocking into the stock means transporting members which have become superfluous as a result of enlargement of the transporting pitch in the assembly line.
On the other hand, instead of providing stock means, a terminal end and a starting end of the assembly line may be connected via a storage line, and a travelling speed of transporting members in the storage line may be controlled according to a change in the transporting pitch in the assembly line so that, when the transporting pitch is enlarged, the superfluous number of transporting members can be kept stored in the storage line by decreasing the travelling speed.